The present invention relates to a hard-material alloy, based on titanium carbide, with alloying constituents from the group of carbides, nitrides and/or borides of the transition metals of secondary groups IVb, Vb, and VIb of the periodic system, said alloy being produced by a melting process. Said material can also contain finely divided alloying elements such as Cu, Ni, Si, Al, Sc, Fe, and Mn. It is preferred that the range of applications for the material include its use as abrasive grain in abrasive agents, as high-melting point compound in the fire-resistant domain, as anti-abrasive material and/or its use in metal-cutting tools.
Simply put, abrasive agents can be divided into two major groups. In the case of the so-called conventional abrasive agents, carborundum, silicon carbide, or other abrasive agents are used as the abrasive grain; these have been known for a relatively long time, and they can be produced or obtained economically; however, their performance is limited by a lack of hardness or deficient toughness, or other characteristics, such as chemical reactivity, amongst others that are unfavourable properties for the abrasion process. For this reason, recently, more widespread use has been made of the so-called super abrasives such as diamonds and cubic boron nitride; these are characterized by extreme hardness andxe2x80x94as a result of thisxe2x80x94extremely good performance as abrasive agents. The disadvantage of super abrasives are their very high production costs associated with the abrasive grain itself. These costs range from one thousand to ten thousand times the production costs for conventional abrasive grains. This notwithstanding, for certain applications, enhanced performance, reduced machine down times, and lower consumption of abrasive agent result in a favourable price/performance ratio. Nevertheless, the scope of application for high-performance abrasive agents remains limited because of their high production costs.
For this reason, it is the objective of the present invention to describe production of an abrasive grain that is economical to produce but which, at the same time, embodies characteristics of high-performance abrasive agents or possesses other properties that are favourable for the grinding process, so as to close the gap between high-performance abrasive grains and conventional abrasive grains. At the same time, it should be possible to use the abrasive grain for the widest possible range of applications.
In addition to extreme hardness, an ideal abrasive grain should also possess other characteristics such as great toughness, resistance to heat and chemicals, good thermal conductivity, etc.
If one considers hardness alone, carbides, nitrides, borides, or mixtures thereof come closest to the high-performance abrasive grains. Thus, for example, the carbides, nitrides, borides of the elements of the transition groups possess a very metallic character, and are characterized by great hardness and high melting points. With regards to their composition, these compoundsxe2x80x94unlike their stoichiometric formal compositionxe2x80x94are stable over a wide ranges. Even though the use of hard materials as abrasive grains or as abrasive agents has been discussed in the literature, their commercial use as grinding abrasive has not been known to any great extent.
In the usual course of events, the hard materials are produced by direct conversion from the elements; by reduction of the oxide powder in the presence of carbon and nitrogen compounds, gases that contain nitrogen or compounds that contain boron; by solid body/gas reactions or by gas-phase reactions (e.g., from halogens by conversion with organo-metallic compounds). In all of the cases mentioned above, the hard material occurs as finely divided powder with grain sizes in the lower xcexcm range. However, a dense, pore-free, and relatively coarse grain is required for most uses as grinding agent. In order to obtain such a dense, pore-free, and relatively coarse grain, the powder has to be consolidated and sintered in the presence of sintering agents and/or binding agents. However, because of their high content of binding agent, sintered bodies manufactured in this way are still of only limited hardness or, in their pure form, are so brittle that they are not suitable as abrasive grain.
For these reason, up to now, hard materials have only been used in the abrasive-agents industry in finely divided form, as burnishing and polishing agents.
DE-OS 2 842 042 and DE-OS 2 646 206 disclose abrasive agent particles based on TiC, TaC, and ZrC, whose basic matrices contain titanium boride particles. The disadvantage of these materials, which are consolidated by hot-pressing, is the costly production methods that are involved, and their residual porosityxe2x80x94despite the costly processxe2x80x94both of which make it difficult to ensure economical and widespread use in the grinding process. It is not known whether or not materials of this kind can be used in any notable quantities as abrasive agents in the abrasive-agent branch.
DE-PS 1 049 290 also discusses the fact that titanium carbide in combination with other carbides has been used for abrasive purposes. At the same time, however, mention is made of the fact that it is of only limited use because of its brittleness.
In addition to the previously defined objective, it is a further objective of the present invention to describe the preparation of hard materials for abrasive purposes, said hard materials not displaying the disadvantages of the materials that form the prior art, as discussed heretofore.
This objective has been achieved by the present invention.
Most surprisingly, it has been found that hard material alloys based on titanium carbide, produced by a melting process, are extremely hard and are also relatively tough, so that they can be used successfully as abrasive grain.
It is preferred that oxides or mixtures of oxides be used as raw materials, these having been converted by lampblack, metals, or other reducing agents in the presence of compounds that contain carbon or boron to form the corresponding carbides or borides. In the same way, however, it is also possible to use the appropriate hard-material powder that corresponds to the alloy directly, or to use other compounds or the metals themselves as the starting materials. The raw-materials mixtures are advantageously smelted in arc or plasma furnaces, in air or an inert atmosphere, as desired. In order to produce nitrides or carbon nitrides, it is advantageous to perform this work in a nitrogen atmosphere.
A further way to produce the hard material alloys is by so-called SHS synthesis (Self-Propagating High Temperature Synthesis), in which the high temperatures required for melting or reaction are generated in situ by appropriate exothermic reactions of the starting materials.
The advantage of the production methods by way of the smelt or the SHS method is that no metal-binding phase is needed in order to produce a dense body of the hard-material alloy or of the hard material, although this does not preclude the fact that the metallic phases can be used and contained as alloying components.
The structure of the hard-material alloy, which exerts a major influence on grinding performance, is formed directly from the smelt and can, if necessary, be optimized by subsequent heat treatment. Depending on the conditions under which cooling takes place, and the selection of the starting materials, the primary crystals of the hard-material alloy will be of a diameter from one to several hundred xcexcm. In the case of eutectic composition, and depending on subsequent annealing it is, however, possible to obtain a much finer structure. As product, one obtains a smelted hard-material regulus that can be formed into the desired grain size or grain by subsequent reduction and screening, for use in abrasive agents.
As a finely divided powder, the hard-material alloy according to the present invention can be also be used as starting material for producing shaped parts for the refractories industry, for manufacturing abrasive parts, and for cutting tools.
The present invention will be described in greater detail below on the basis of examples, without necessarily being restricted thereto.